1. Field of the Invention
The present invention relates to an ink set, an ink-jet recording process, an ink-jet recording apparatus, and a method for alleviating the formation of solids within a common recovery-system unit.
2. Related Background Art
There have heretofore been reported black inks using self-dispersing carbon black for forming black-colored images excellent in optical density, print quality and fastness properties such as water fastness and light fastness on, in particular, plain paper in recording by an ink-jet system and also satisfying ejection stability and dispersion stability, ink sets by which no bleeding occurs at boundaries between an image printed with a black ink and an image printed with a color ink, and ink-jet recording processes and ink-jet recording apparatus or instruments using such an ink set.
For example, Japanese Patent Application Laid-Open No. 8-3498 describes self-dispersing carbon black capable of being stably dispersed without using any dispersing agent by introducing a water-soluble group in the surface of carbon black. Japanese Patent Application Laid-Open No. 10-183046 reports ink sets comprising a black ink using self-dispersing carbon black having a cationic group at the surface thereof and color inks each using an anionic dye.
In ink-jet recording apparatus such as ink-jet printers, ejection failure may occur in some cases due to mixing of bubbles, dust, recession of a meniscus in an ink ejection orifice at the tip of a nozzle and/or the like upon printing by a recording head, resulting in a failure to provide a clear print. In order to prevent such a situation, it is conducted to recover ejection of an ink from a nozzle to a normal state and to preliminarily discharge an ink so as to stabilize the ejection properties of the ink from the nozzle in advance. When such an operation for normally ejecting an ink from a nozzle, which is not conducted for printing as the original object, is performed in an ink-jet recording apparatus, it is general to conduct the operation of capping a region of each ink-ejection orifice with a cap and then sucking the ink present in the nozzle and the like through the cap by means of a sucking means such as a pump. The ink sucked is gathered as a waste ink in an ink reservoir through, for example, a tube for suction and recovery.
The ink reservoir is generally so constructed that a member having high water-absorbing capacity is contained in a cartridge for receiving a waste ink in order to prevent the ink from leaking from a feed opening of the ink reservoir (hereinafter referred to as xe2x80x9cwaste ink-absorbing memberxe2x80x9d). As the member having high water-absorbing capacity, which is usable as the waste ink-absorbing member, there may be used, for example, a high-density fibrous structure such as blotting paper or felt, a foam-molded material of polyurethane, PVA (polyvinyl alcohol) or the like, or the absorbing member comprising a water-absorbing polymer. The same shall apply to a member (hereinafter referred to as xe2x80x9cin-cap absorbing memberxe2x80x9d) contained in the cap for capping the ejection opening face of the recording head.
In the course of a variety of evaluation under such circumstances as described above, which has been conducted by charging a black ink comprising self-dispersing carbon black into an ink-jet printer, it has been found that the ink may come not to be gradually absorbed in the waste ink-absorbing member in some cases. In the very case where such an ink set comprising a black ink using self-dispersing carbon black having a cationic group at the surface thereof and color inks using an anionic dye as described in Japanese Patent Application Laid-Open No. 10-183046 is used, and an ink-jet recording apparatus has a cap for capping an ejection opining face of a recording head, a pump communicating with the interior of the cap and being capable of sucking inks contained in the cap, a waste ink-absorbing member, a passageway through which waste inks are operated, and the like for common use for the black ink and the color inks, solids such as aggregates and deposits have been formed in the interiors of the cap, pump, waste ink-absorbing member and the like (hereinafter referred to as xe2x80x9crecovery-system unitxe2x80x9d) in some cases, resulting in a failure to reliably recover waste inks without staining the interior of the apparatus.
Although the common use of recovery-system unit for the black ink and the color inks is an extremely useful means for miniaturizing an ink-jet recording apparatus, the present inventors have been led to the conclusion that the adoption of this construction requires technical development for permitting the reliable recovery of the waste inks over a long period of time.
It is therefore an object of the present invention to provide an ink set which can prevent the formation of solids within a recovery-system unit of an ink-jet recording apparatus, and permits the reliable recovery of waste inks over a long period of time and the stable provision of high-quality prints.
Another object of the present invention is to provide an ink-jet recording process which permits the stable provision of high-quality prints.
A further object of the present invention is to provide a recording unit and an ink cartridge which can prevent the formation of solids within a recovery-system unit of an ink-jet recording apparatus, and permits the reliable recovery of waste inks over a long period of time and stable recording with high quality over a long period of time.
A still further object of the present invention is to provide an ink-jet recording apparatus which permits the stable provision of high-quality prints.
A yet still further object of the present invention is to provide an ink-jet recording apparatus which permits the stable formation of high-quality color images which scarcely undergo bleeding and staining with waste inks and the like.
A yet still further object of the present invention is to provide a method for alleviating the formation of aggregates and deposits within a recovery-system unit of an ink-jet recording apparatus.
In order to achieve the above objects, the present inventors have carried out various investigations and concluded that the formation of solids in the waste ink-absorbing member is considered to be attributable to the dispersion breaking of self-dispersing carbon black in a black ink comprising pigment particles of the self-dispersing carbon black. More specifically, for example, a hydrophilic group is being bonded to the surface of self-dispersing carbon black, and a counter ion (for example, an ammonium ion) to the hydrophilic group gradually volatilizes during the retention of the ink within the recovery-system unit. As a result, the dispersibility of the self-dispersing carbon black in an aqueous medium is deteriorated, and so the ink is changed to a state liable to form solids, and the solids formed gradually adhere to the recovery-system unit including the waste ink-absorbing member.
On the other hand, the present inventors have carried out an investigation as to the relationship between viscosity and pH in a black ink comprising pigment particles of anionically charged self-dispersing carbon black. As a result, it has been found that the ink exhibits such relationship as shown in (a) in FIG. 7. More specifically, the viscosity of the black ink rapidly increases or lowers as the pH thereof becomes lower or higher. Accordingly, when the pH of the black ink is lowered within a recovery system by some reason, and its viscosity is rapidly increased, only a medium in the ink is absorbed in the waste ink-absorbing member, and so the self-dispersing carbon black remains on the surface of the absorbing member. It is thus considered that the carbon black gradually deposits, so that the absorption of waste inks is prevented. The present inventors have already found that the viscosity of the black ink comprising the self-dispersing carbon black having an anionic hydrophilic group as the hydrophilic group on the surface thereof is lowered in an alkaline region and increased by dispersion breaking in an acid region.
Thus, the present inventors have inferred that to prevent the dispersion breaking of the black ink is useful for preventing the adhesion of solids to the recovery system and found that when the pH of the black ink is adjusted so as not to form solids within the recovery system, the adhesion of solids formed is extremely effectively prevented. The present invention has been led to completion on the basis of these findings.
The above objects can be achieved by the present invention described below.
According to a first aspect of the present invention, there is thus provided an ink set comprising a black ink and a color ink and being suitable for an ink-jet recording apparatus comprising a common recovering means for a black ink-ejecting means and a color ink-ejecting means, wherein the black ink comprises particles of self-dispersing carbon black having at least one anionic hydrophilic group at the surface thereof in an aqueous medium, and the pH of the color inks is higher than the pH of the black ink.
According to another aspect of the present invention, there is also provided an ink-jet recording process, comprising the step of: (i) applying energy to at least one of a black ink and a color ink; (ii) ejecting the ink to which the energy is applied and attaching the ejected ink to a recording medium, the steps (i) and (ii) being conducted with an ink-jet recording apparatus provided with a common recovery means for a black ink-ejecting means and a color ink-ejecting means, wherein the black ink comprises particles of self-dispersing carbon black having at least one anionic hydrophilic group at the surface thereof in an aqueous medium and having a prescribed pH, and the color ink has a pH higher than the pH of the black ink.
According to another aspect of the present invention, there is further provided a recording unit comprising at least two ink-containing portions containing a black ink and a color ink respectively, and an ink-jet head for ejecting the black ink and the color ink respectively, the recording unit being for an ink-jet recording apparatus provided with a common recovery means for a black ink-ejecting orifice and a color ink-ejecting orifice of the ink-jet head, wherein the black ink comprises particles of self-dispersing carbon black having at least one anionic hydrophilic group at the surface thereof in an aqueous medium and having a prescribed pH, and the color ink has a pH higher than the pH of the black ink.
According to another aspect of the present invention, there is still further provided an ink cartridge suitable for use in an ink-jet recording apparatus provided with an ink-jet head ejecting a black ink and a color ink, and a common recovering means for a black ink-ejecting orifice of the ink-jet head and a color ink-ejecting orifice of the ink-jet head comprising at least two ink-containing portions containing the black ink and the color ink, respectively, wherein the black ink comprises particles of self-dispersing carbon black having at least one anionic hydrophilic group at the surface thereof in an aqueous medium and has a prescribed pH, and the color ink has a pH higher than the pH of the black ink.
According to another aspect of the present invention, there is yet still further provided an ink-jet recording apparatus comprising a recording unit which has at least two ink-containing portions and a head portion for ejecting the inks fed from the ink-containing portions in the form of ink droplets by the action of thermal energy, and further a cap for capping ejection opening faces of the head portion, a pump communicating with the interior of the cap and being capable of sucking an ink contained in the cap, and a waste ink-absorbing member for absorbing and preserving the waste ink discharged from the pump, wherein the ink-containing portions respectively contains at least two inks making up the ink set described above.
According to another aspect of the present invention, there is yet still further provided an ink-jet recording apparatus comprising a recording unit which has an ink cartridge equipped with at least two ink-containing portions, and a head portion for ejecting the inks fed from the ink-containing portions in the form of ink droplets by the action of thermal energy, and further a cap for capping ejection opening faces of the recording head, a pump communicating with the interior of the cap and being capable of sucking an ink contained in the cap, and a waste ink-absorbing member for absorbing and preserving the waste ink discharged from the pump, wherein the ink-containing portions separately contains at least two inks making up the ink set described above.
According to another aspect of the present invention, there is yet still further provided an ink-jet recording apparatus comprising a recording unit which has at least two ink-containing portions respectively containing at least two inks of different colors and a head portion for ejecting the inks fed from the ink-containing portions, a cap for capping together ejection opening faces for said at least two inks provided in the head portion, a pump for sucking the inks in the head portion through the cap, and an ink-absorbing member for preserving said at least two inks sucked by the pump, wherein said two inks of different colors are a black ink comprising self-dispersing carbon black having at least one anionic hydrophilic group at the surface thereof in an aqueous medium, and a color ink having a pH higher than the pH of the black ink.
According to further aspect of the present invention, there is yet still further provided an ink-jet recording apparatus comprising a plurality of ink cartridges separately containing at least two inks of different colors, a head portion, in which the ink cartridges are detachably installed and from which the inks contained in ink-containing portions of the respective ink cartridges are ejected, a cap for capping together ejection opening faces for said at least two inks provided in the head portion, a pump for sucking the inks in the head portion through the cap, and an ink-absorbing member for preserving said at least two inks sucked by the pump, wherein said two inks of different colors are a black ink comprising self-dispersing carbon black having at least one anionic hydrophilic group at the surface thereof in an aqueous medium, and a color ink having a pH higher than the pH of the black ink.
According to still further aspect of the present invention, there is yet still further provided a method for alleviating the formation of solids within a common recovery-system unit of an ink-jet recording apparatus which is provided with a black ink comprising self-dispersing carbon black and a color ink and has a recovery-system unit for common use for the black ink and the color ink, which comprises using a color ink having a pH higher than the pH of the black ink as the color ink.
According to the above-described embodiments of the present invention, even when an ink-jet recording apparatus has a recovery system for common use for a plurality of inks, the formation of solids within the recovery system can be prevented. As a result, a black-colored print having excellent water fastness can be provided. There is also brought about an effect that waste inks produced by preliminary ejection of inks and a recovery operation such as recovery by suction or pressurization, which are necessary for printing using an in-jet recording apparatus, can be reliably recovered stably over a long period of time without staining the interior of the apparatus. Further, an effect that an ink-jet recording apparatus can be miniaturized can be brought about by the common use of a recovery system for a black ink and color inks.